It is important to know what browsers are being used on our sites since you are the ones building them. Here is a list of the top browsers, by percentage. One thing to note, internally we use about 50/50 Chrome and Firefox. With more Firefox 4 users than Firefox 3.x. Compare that with our users statistics.
42.0% - IE 8
12.5% - IE 7
12.0% - Safari(multiple versions)
12.0% - Firefox 3.x
6.5% - Chrome
5.0% - Firefox 4
5.0% - IE 9
5.0% - other (mobile, ie6, etc)
What this shows is we are actually building in some of the least used browsers by our users. This is a reminder that if you are making changes to a site, or creating a new one, IE is still king amongst our users. IE 8 needs to have plenty of time committed to testing your site against. Any changes you make to markup or widgets or styles must be checked in IE 8 to make sure they are working correctly for a significant portion of our user base.
Its also worth noting that Firefox 4 and IE 9 have been experiencing incredibly fast adoption rates and will likely surpass Chrome very soon, with IE 9 likely moving past Safari and Firefox 3.x before the end of the year. IE 9 has some drastic changes in the way javascript is rendered and in it's approach to web standards. I recommend every Website Manager have IE 9 installed, and utilize its developer tools to switch between IE 7 / IE 8 / IE 9 modes for browser testing.

For those that don't know, there are multiple versions of HTML. A at the top of a page tells the browser what version of HTML the page was created for. Our sites tend to be either HTML 4.01 Transitional or XHTML 1.0 Transitional. There are several differences between the two. On the horizon is HTML 5 which has a bunch of new functionality included and is supported in some way by most of the latest browser versions.
An important step in making sure our sites are cross browser compatible, is to define a and make sure our HTML follows the guidelines of that doctype. To check how your sites perform against your chosen doctype, you can validate a site by going to
http://validator.w3.org/
and entering the URL. A completely valid document is incredibly important for cross browser performance. Unfortunately, we don't have very valid markup. We tend to freely mix HTML and XHTML and leave out a lot of requirements and let the browsers make up the difference. We need to start focusing on these details and making sure we produce the highest quality websites.
Here is some insight into the average performance of our top 5 traffic'd sites(excluding World Journal).
320 Errors, 140 Warnings
With one site having 503 errors. It is important to note that most of these errors are actually repeats of the same error, or caused by errors higher up in the document that just cascade into making the rest of the document seem to have more errors.
A few tips for cleaning up errors.
1) All widgets should have unique instance names in a page, no two widgets on the same page should have the same instance name. This gives them unique DOM IDs
2) Any widget in the section of the document(think @@content@@ widgets) need to use :bare => true, and :naked => true to avoid bringing

tags into the section.
3) Use doctypes for XHTML instead of HTML and focus on that single version, then make sure all self closing tags include a '/' like and
4) If you include a